Cancer is a major public health concern in the United States and around the world. It is estimated that over 1 million new cases of invasive cancer will be diagnosed in the United States in 1998. The most prevalent forms of the disease are solid tumors of the lung, breast, prostate, colon and rectum. Cancer is typically diagnosed by a combination of in vitro tests and imaging procedures. The imaging procedures include X-ray computed tomography, magnetic resonance imaging, ultrasound imaging and radionuclide scintigraphy. Frequently, a contrast agent is administered to the patient to enhance the image obtained by X-ray CT, MRI and ultrasound, and the administration of a radiopharmaceutical that localizes in tumors is required for radionuclide scintigraphy.
Despite the variety of imaging procedures available for the diagnosis of cancer, there remains a need for improved methods. In particular, methods that can better differentiate between cancer and other pathologic conditions or benign physiologic abnormalities are needed. One means of achieving this desired improvement would be to administer to the patient a metallopharmaceutical that localizes specifically in the tumor by binding to a receptor expressed only in tumors or expressed to a significantly greater extent in tumors than in other tissue. The location of the metallopharmaceutical could then be detected externally either by its imageable emission in the case of certain radiopharmaceuticals or by its effect on the relaxation rate of water in the immediate vicinity in the case of magnetic resonance imaging contrast agents.
Previous efforts to achieve these desired improvements in cancer imaging and treatment have centered on the use of radionuclide labeled monoclonal antibodies, antibody fragments and other proteins or polypeptides that bind to tumor cell surface receptors. The specificity of these radiopharmaceuticals is frequently very high, but they suffer from several disadvantages. First, because of their high molecular weight, they are generally cleared from the blood stream very slowly, resulting in a prolonged blood background in the images. Also, due to their molecular weight they do not extravasate readily at the site of the tumor and then only slowly diffuse through the extravascular space to the tumor cell surface. This results in a very limited amount of the radiopharmaceutical reaching the receptors and thus very low signal intensity in imaging.
Alternative approaches to cancer imaging have involved the use of small molecules, such as peptides, that bind to tumor cell surface receptors. An In-111 labeled somatostatin receptor binding peptide, In-111-DTPA-D-Phe1-octeotide, is in clinical use in many countries for imaging tumors that express the somatostatin receptor (Baker, et al. Life Sci., 1991, 49, 1583–91 and Krenning, et al., Eur. J. Nucl. Med., 1993, 20, 716–31). Higher doses of this radiopharmaceutical have been investigated for potential treatment of these types of cancer (Krenning, et al., Digestion, 1996, 57, 57–61). Several groups are investigating the use of Tc-99m labeled analogs of In-111-DTPA-D-Phe1-octeotide for imaging and Re-186 labeled analogs for therapy (Flanagan, et al., U.S. Pat. No. 5,556,939, Lyle, et al., U.S. Pat. No. 5,382,654, and Albert et al., U.S. Pat. No. 5,650,134).
Angiogenesis is the process by which new blood vessels are formed from pre-existing capillaries or post capillary venules; it is an important component of a variety of physiological processes including ovulation, embryonic development, wound repair, and collateral vascular generation in the myocardium. It is also central to a number of pathological conditions such as tumor growth and metastasis, diabetic retinopathy, and macular degeneration. The process begins with the activation of existing vascular endothelial cells in response to a variety of cytokines and growth factors. Tumor released cytokines or angiogenic factors stimulate vascular endothelial cells by interacting with specific cell surface receptors for the factors. The activated endothelial cells secrete enzymes that degrade the basement membrane of the vessels. The endothelial cells then proliferate and invade into the tumor tissue. The endothelial cells differentiate to form lumens, making new vessel offshoots of pre-existing vessels. The new blood vessels then provide nutrients to the tumor permitting further growth and a route for metastasis.
Under normal conditions, endothelial cell proliferation is a very slow process, but it increases for a short period of time during embryogenesis, ovulation and wound healing. This temporary increase in cell turnover is governed by a combination of a number of growth stimulatory factors and growth suppressing factors. In pathological angiogenesis, this normal balance is disrupted resulting in continued increased endothelial cell proliferation. Some of the proangiogenic factors that have been identified include basic fibroblast growth factor (bFGF), angiogenin, TGF-alpha, TGF-beta, and vascular endothelium growth factor (VEGF). While interferon-alpha, interferon-beta and thrombospondin are examples of angiogenesis suppressors.
The proliferation and migration of endothelial cells in the extracellular matrix is mediated by interaction with a variety of cell adhesion molecules (Folkman, J., Nature Medicine , 1995, 1, 27–31). Integrins are a diverse family of heterodimeric cell surface receptors by which endothelial cells attach to the extracellular matrix, each other and other cells. The integrin αvβ3 is a receptor for a wide variety for a wide variety of extracellular matrix proteins with an exposed tripeptide Arg-Gly-Asp moiety and mediates cellular adhesion to its ligand: vitronectin, fibronectin, and fibrinogen, among others. The integrin αvβ3 is minimally expressed on normal blood vessels, but is significantly upregulated on vascular cells within a variety of human tumors. The role of the αvβ3 receptors is to mediate the interaction of the endothelial cells and the extracellular matrix and facilitate the migration of the cells in the direction of the angiogenic signal, the tumor cell population. Angiogenesis induced by bFGF or TNF-alpha depend on the agency of the integrin αvβ3, while angiogenesis induced by VEGF depends on the integrin αvβ3 (Cheresh et. al., Science, 1955, 270, 1500–2). Induction of expression of the integrins α1β1 and α2β1 on the endothelial cell surface is another important mechanism by which VEGF promotes angiogenesis (Senger, et. al., Proc. Natl. Acad, Sci USA, 1997, 84, 13612–7).
Angiogenic factors interact with endothelial cell surface receptors such as the receptor tyrosine kinases EGFR, FGFR, PDGFR, Flk-1/KDR, Flt-1, Tek, tie, neuropilin-1, endoglin, endosialin, and Axl. The receptors Flk-1/KDR, neuropilin-1, and Flt-1 recognize VEGF and these interactions play key roles in VEGF-induced angiogenesis. The Tie subfamily of receptor tyrosine kinases are also expressed prominently during blood vessel formation.
Thus, it is desirable to provide tumor or new vasculature imaging agents which do not suffer from poor diffusion or transportation, possible immunologic toxicity, limited availability, and/or a lack of specificity.
The detection, imaging and diagnosis of a number of cardiovascular diseases need to be improved, including restenosis, atherosclerosis, myocardial reperfusion injury, and myocardial ischemia, stunning or infarction. It has recently been determined that in all of these disease conditions, the integrin receptor αvβ3 plays an important role.
For example, in the restenosis complication that occurs in ˜30–50% of patients having undergone angioplasty or stent placement, neointimal hyperplasia and ultimate reocclusion is caused by aggressively proliferating vascular smooth muscle cells that express αvβ3. (Cardiovascular Res., 1997, 36, 408–428; DDT, 1997, 2, 187–199; Current Pharm. Design, 1997, 3, 545–584)
Atherosclerosis proceeds from an initial endothelial damage that results in the recruitment and subintimal migration of monocytes at the site of the injury. Growth factors are released which induce medial smooth muscle cells to proliferate and migrate to the intimal layer. The migrating smooth muscle cells express αvβ3.
In reperfusion injury, neutrophil transmigration is integrin dependent and the integrins moderate initial infiltration into the viable border zone. The induction of α5β1, α4β1 and αvβ5 in infiltrating neutrophils occurs within 3 to 5 hours after reperfusion as neutrophils move from the border zone to the area of necrosis. (Circulation, 1999, 100, I-275)
Acute or chronic occlusion of a coronary artery is known to result in angiogenesis in the heart as native collateral vessels are recruited to attempt to relieve the ischemia. However, even a gradual occlusion usually results in areas of infarction as the resulting angiogenesis is not sufficient to prevent damage. Cardiac angiogenesis has been associated with increased expression of the growth factors VEGF and FGF and the upregulation of the growth factor receptors flt-1 and flk-1/KDR. (Drugs, 1999, 58, 391–396).